Acinetobacter baumannii is one of the most frequently isolated bacteria from patients with ventilator-associated pneumonia and causes a diverse number of infections, particularly in highly susceptible individuals in intensive care unit. The rapid development of antibiotic resistance by A. baumannii has led to a significant decrease in our ability to effectively treat infections. Consequently, there has been a considerable expansion in research directed towards alternative preventative measures and treatments. Understanding the mechanisms of A. baumannii pathogenesis will improve our capacity for successful therapeutic intervention and may identify bacterial factors that can be targeted for development of new antimicrobials or vaccines. Calprotectin (CP) is a host protein present at sites of inflammation where it displays pro-inflammatory activity. CP also exhibits antimicrobial activity through sequestration of the essential metals zinc (Zn) and manganese (Mn). CP- mediated Zn chelation inhibits A. baumannii growth, and during A. baumannii pneumonia, the loss of CP enhances bacterial proliferation and dissemination. It is well-established that bacteria sense and respond to nutrient starvation via expression of specialized acquisition systems, modification of metabolic processes, and alterations to the metalloproteome. Bacteria that lack these capabilities are generally unable to establish a successful infection. Therefore, A. baumannii must employ strategies to acquire Zn within the host. In experiments to identify bacterial factors required for A. baumannii adaptation to CP-mediated Zn starvation, we discovered an A. baumannii Zn acquisition system that we have named ZnuABC. This system is under the control of a Zn-responsive regulator, which we have named Zur. Further in silico analysis of the genomic region upstream of the genes encoding Zur and ZnuABC uncovered a conserved sequence called a zur box, to which Zur binds to repress downstream gene expression. A search for this consensus sequence within the A. baumannii genome identified additional genes that may be regulated by Zur and Zn. We hypothesize that following infection, CP generates a Zn-starved environment, and A. baumannii responds through the up- regulation of a Zn acquisition system and other Zn-responsive genes that are essential for pathogenesis. This proposal addresses our hypothesis in the following series of three integrated Specific Aims. Aim 1: Determine the role of CP-mediated Zn starvation to A. baumannii pneumonia. Aim 2: Define the A. baumannii response to Zn-limiting conditions. Aim 3: Elucidate the functions of two Zn-regulated genes in A. baumannii Zn homeostasis. Together these studies will improve our understanding of host nutritional immunity during bacterial pneumonia and define how A. baumannii subverts this host defense to cause disease. Furthermore, characterization of these mechanisms will lay the groundwork for the development of improved therapeutics in the face of the rapidly decreasing efficacy of the current treatments for A. baumannii infection.